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171.
We consider a class of fourth‐order nonlinear diffusion equations motivated by Tumblin and Turk's “low‐curvature image simplifiers” for image denoising and segmentation. The PDE for the image intensity u is of the form where g(s) = k2/(k2 + s2) is a “curvature” threshold and λ denotes a fidelity‐matching parameter. We derive a priori bounds for Δu that allow us to prove global regularity of smooth solutions in one space dimension, and a geometric constraint for finite‐time singularities from smooth initial data in two space dimensions. This is in sharp contrast to the second‐order Perona‐Malik equation (an ill‐posed problem), on which the original LCIS method is modeled. The estimates also allow us to design a finite difference scheme that satisfies discrete versions of the estimates, in particular, a priori bounds on the smoothness estimator in both one and two space dimensions. We present computational results that show the effectiveness of such algorithms. Our results are connected to recent results for fourth‐order lubrication‐type equations and the design of positivity‐preserving schemes for such equations. This connection also has relevance for other related fourth‐order imaging equations. © 2004 Wiley Periodicals, Inc.  相似文献   
172.
We have measured the neutron spin asymmetry A(n)(1) with high precision at three kinematics in the deep inelastic region at x=0.33, 0.47, and 0.60, and Q(2)=2.7, 3.5, and 4.8 (GeV/c)(2), respectively. Our results unambiguously show, for the first time, that A(n)(1) crosses zero around x=0.47 and becomes significantly positive at x=0.60. Combined with the world proton data, polarized quark distributions were extracted. Our results, in general, agree with relativistic constituent quark models and with perturbative quantum chromodynamics (PQCD) analyses based on the earlier data. However they deviate from PQCD predictions based on hadron helicity conservation.  相似文献   
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Chiral copper complexes of C2-symmetrical phosphoroamidites were found to be highly effective catalysts for both kinetic resolution and novel desymmetrization reactions of new methylidene epoxycycloalkanes.  相似文献   
175.
We calculate the scaling behavior of the second-kind self-similar blow-up solution of an aggregation equation in odd dimensions. This solution describes the radially symmetric finite-time blowup phenomena and has been observed in numerical simulations of the dynamic problem. The nonlocal equation for the self-similar profile is transformed into a system of ODEs that is solved using a shooting method. The anomalous exponents are then retrieved from this transformed system.  相似文献   
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Mucin glycoproteins contribute to a wide range of cell-surface phenomena. Their dense glycosylation is believed to confer structural rigidity as well as molecular extension beyond the glycocalyx, crucial to interaction with the cellular environment. However, controlled investigations of the relationships between glycosylation, rigidity, and extension of membrane-bound mucins or similar macromolecules are lacking, largely because of the absence of tractable experimental models. We have therefore made use of recently developed synthetic mucin mimetics, in which the core alpha-GalNAc monosaccharides of natural mucins are conjugated to a lipidated polymer backbone and anchored to fluid, solid-supported lipid membranes, and fluorescence interference contrast microscopy, an optical technique that provides nanometer-scale topographic information about objects near a reflective interface, to measure the orientation of the mucin mimics relative to the membrane plane. Data from two independent probes, fluorophores conjugated directly to the polymer backbone and fluorescent proteins bound to the sugar groups, unexpectedly show that the mucin mimic molecules lie flat along the membrane. Rigidity and core glycosylation are therefore insufficient to ensure molecular projection outward from a membrane surface.  相似文献   
178.
Sulfolipid-I (SL-I) is an abundant metabolite found in the cell wall of Mycobacterium tuberculosis that is comprised of a trehalose 2-sulfate core modified with four fatty acyl substituents. The correlation of its abundance with the virulence of clinical isolates suggests a role for SL-I in pathogenesis, although its biological functions remain unknown. Here we describe the synthesis of a SL-I analogue bearing unnatural lipid substituents. A key feature of the synthesis was application of an intramolecular aglycon delivery reaction to join two differentially protected glucose monomers, one prepared with a novel alpha-selective glycosylation. The route developed for the model compound can be readily extended to the synthesis of native SL-I as well as additional analogues for use in the investigation of SL-I's functions.  相似文献   
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Sletten EM  Bertozzi CR 《Organic letters》2008,10(14):3097-3099
Biomolecules labeled with azides can be detected through Cu-free click chemistry with cyclooctyne probes, but their intrinsic hydrophobicity can compromise bioavailability. Here, we report the synthesis and evaluation of a novel azacyclooctyne, 6,7-dimethoxyazacyclooct-4-yne (DIMAC). Generated in nine steps from a glucose analogue, DIMAC reacted with azide-labeled proteins and cells similarly to cyclooctynes. However, its superior polarity and water solubility reduced nonspecific binding, thereby improving the sensitivity of azide detection.  相似文献   
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